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Yard and Garden: Pruning Trees

AMES, Iowa – There are many valid reasons for pruning trees and shrubs. The most important reasons include pruning for safety, health and appearance. Iowa State University Extension and Outreach horticulturists discuss the best time to prune common Iowa trees. To have additional tree, yard and garden questions answered, contact Hortline at 515-294-3108 or email hortline@iastate.edu. Find answers to common yard and garden questions at the ISU Horticulture Department’s Yard and Garden FAQs website, http://expert.hort.iastate.edu/.

When is the best time to prune oak trees?

Winter (December through February) is the best time to prune oak trees in Iowa. Pruning oak trees in winter greatly reduces the risk of an oak wilt infection.

Oak wilt is a fungal disease that is lethal to many oaks. It can be spread from infected trees to healthy trees by sap-feeding beetles (“picnic bugs”). Oak wilt infections occur most commonly in spring and early summer. The sap-feeding beetles are very active in spring and early summer. During this same time, oak wilt infected trees are producing masses of spore-producing fungal material (spore mats). These mats release a fruity odor that attracts sap-feeding beetles and other insects. As the beetles feed on the spore mats, spores often accumulate on the surface of their bodies. Sap that forms at the surface of pruning cuts made in spring or early summer may attract sap-feeding beetles that may have been previously feeding on an oak wilt infested tree. As the beetles feed on the sap of the pruning cut, fungal spores get into the fresh wound, infecting the tree. Pruning oak trees in winter greatly reduces the risk of an oak wilt infection as the beetles and fungal mats are not present at that time of year.

If an oak tree must be pruned in spring or summer (such as after a storm), immediately apply latex house paint to the pruning cuts to avoid attracting sap-feeding beetles to the wounds.

When is the best time to prune shade trees?

February through March is generally regarded as the best time to prune most shade trees. The absence of foliage at this time of year gives the individual a clear view of the tree and allows the selection and removal of appropriate branches. Also, the walling-off or compartmentalization of wounds occurs most rapidly just prior to the onset of growth in spring. Oaks are an exception. The winter months – December, January and February – are the best time to prune oak trees.

Large amounts of sap often flow from pruning cuts on maple, birch and elm when pruned in late winter. However, the loss of sap doesn’t harm the trees. The trees won’t “bleed” to death. Eventually the flow of sap will slow and stop.

Shade trees can also be pruned in midsummer. If possible, avoid pruning shade trees in spring as they are leafing out. In spring, the tree’s energy reserves are low and the bark tears easily. Another poor time to prune is during leaf drop in fall.

When should I prune my fruit trees?

Late February to early April is the best time to prune fruit trees in Iowa. Summer pruning of fruit trees is generally not recommended. However, water sprouts (rapidly growing shoots that often develop just below a pruning cut) can be removed in June or July.